Arrhythmias and sinus tachycardia are often seen after open heart surgery, or in the course of an acute myocardial infarct. Pharmacologic agents are often ineffective to control these arrhythmias, or may have toxic or undesirable side effects. Nonpharmacologic methods, such as carotid sinus pressure and various types of pacing, (rapid atrial pacing, paired or coupled pacing, and times extrasystoles) are often ineffective, especially in febrile states. The objective of this program is to develop an alternative therapeutic technique to treat supraventricular tachyarrhythmias, and sinus tachycardia. During open heart surgery, we have seen that the heart rate is easily and quickly slowed by causing slight local cooling of the SA node with the application of cold sponges. We have evaluated this effect initially in an animal model using an open chest preparation. Small cooling probes (thermodes) were designed to operate in conjunction with miniature thermocouples to cool and monitor the SA node. Initial data demonstrates the feasibility for varying SA node temperature, in order to control heart rate within a clinically useful range. It is expected that complete data on this phenomenon will lead to a simple technique for controlling arrhythmias which will have wide clinical use.